Liquid crystal materials for electro-optical indicators frequently contain one or more optically active additives for the induction of a chiral structure. For example, liquid crystal phases for indicators having a twisted nematic structure can be doped with optically active additives, e.g. in order to avoid a reversal of the direction of twisting (reverse twist) in TN cells (twisted-nematic) or in order to produce a sufficient twisting in cells having a highly twisted nematic structure, such as STN cells (super twisted-nematic), SBE cells (super birefringence effect) or OMI cells (optical mode interference). Further, cholesteric liquid crystals for phase-change cells can preferably consist of a nematic basic material and one or more optically active dopants and ferroelectric liquid crystals for indicators based on chiral tilted smectic phases can preferably consist of a material having a tilted smectic phase and one or more optically active dopants.
The electro-optical characteristics of liquid crystal indicators are temperature-dependent, which can be disadvantageous especially in the case of multiplex operation. It is, however, known that this temperature dependence can be compensated for at least partly by the addition of a chiral dopant which induces a lower pitch with increasing temperature. Such an inverse temperature dependence has hitherto been found only for a few compounds. It can, however, also be achieved by the use of at least 2 chiral dopants which have a different relative temperature dependence and induce a different twisting direction (U.S. Pat. No. 4,264,148). This usually requires a relatively high amount of chiral dopants.
Cholesteric liquid crystals reflect light essentially only in a wavelength range for which the wavelength is approximately equal to the helical pitch. The spectral width of this reflected light can be varied by a suitable choice of the liquid crystal. The reflected light is completely circularly polarized. The direction of rotation of the reflected light depends on the direction of rotation of the cholesteric helical structure. The light which is circularly polarized in the opposite direction is transmitted unimpaired. These properties can be utilized for the production of optical filters, polarizers, analyzers etc. Further, cholesteric liquid crystals are also variously used for thermochromic applications.
Cholesteric liquid crystals for the above applications can consist of a nematic or cholesteric basic material and one or more chiral dopants, which permits a simple adjustment of the desired helical pitch.
In order to produce cholesteric mixtures having a pitch in the range of the wavelength of visible light, the chiral dopant should have a twisting capacity which is as high as possible and should have good solubility in usual liquid crystal materials. Furthermore, the chiral dopants should have sufficient stability. They should have good compatibility with the mesophase type of liquid crystal material and should not severely restrict the mesophase range.